1. Field of Invention
The present invention relates to a method of fabricating an anti-reflection coating (ARC) layer. More particularly, the present invention relates to a method of stabilizing the ARC layer.
2. Description of Related Art
During a photolithography module of an integrated circuit (IC) fabrication, masks are often made to allow several patterns to be transferred onto a wafer. Each of the masks is usually formed by irradiating a layer of photoresist with light, while unwanted areas of photoresist are removed in a development step to leave behind the desired mask. However, those masks have known to be formed over many reflective materials such as aluminum or polycrystalline silicon which often reflect light into regions of photoresist that are not intended to be irradiated. The unintentional irradiation causes the resulting mask to be inaccurate by causing reflective notching in the mask or inaccurate line width.
Reflective notching on highly reflective substrates and linewidth (or critical dimension, CD) variations due to topography and film thickness non-uniformity have been a difficult problem for semiconductor manufacturers. A useful method such as an anti-reflective coating (ARC) has been developed to suppress reflectivity, which method involves applying an ARC layer over the reflective material before the application of the photoresist. Conventionally, different types of ARC layers, including organic and inorganic ones have been developed. Since problems, such as reacting with the underlying layers have occurred when the organic ARC layers are used, most of the process preferentially adopts the inorganic ARC layer instead.
Typically, an inorganic ARC layer, such as silicon oxy-nitride (SiON) layer is used to prevent the above-mentioned problem. FIGS. 1A and 1B are cross-sectional, schematic diagrams illustrating the fabrication of ARC layer during a conventional photolithography module, in which a substrate 100 is provided with a gate oxide layer 102, a polysilicon layer 104, and a SiON layer 106 formed in sequence thereon. A photoresist layer 108 is then formed over the SiON layer 106. Next, an exposure step is performed where a selected light source that passes through a photomask 110 irradiates the photoresist layer 108 to initiate a photochemical transformation, which replicates a desired mask pattern in the photoresist layer 108a. A development step is then performed to transfer the mask pattern onto the wafer.
It has recently been known that reflectance of the SiON layer 106 fluctuates over time, while the reflectance fluctuation of the SiON layer 106 makes it difficult to control the exposure conditions during the photolithographic process. This leads to reflective notching in the photomask 110, where the desired pattern is distorted. Subsequently, a part of the photoresist layer 108b is removed, leading to formation of IC device with non-uniform CD. A quality control step, such as after develop inspection (ADI) is executed to check whether the specification of the photolithography process is met, while the photolithography module is stripped and reworked to compensate any abnormalities that causes permanent damage to the wafer in the subsequent process. However, measurements of CD taken during ADI indicate that the reflectance of the SiON layer 106 still fluctuates under the same exposure conditions even after reworks. Therefore, it is necessary to develop a method for stabilizing the specificity of the SiON layer.